This invention relates to a photocathode for electroradiographic and electrofluoroscopic apparatus in general and more particularly to such a photocathode which has improved sensitivity and resolution.
There is a trend in the field of medical technology to replace X-ray film, which is still generally used in diagnosis, by a more cost effective recording method, which also saves raw materials. Starting out from a method commonly used in xerographic copying technology, to develop and fix an electrostatic image by means of a contrast powder, one attempts in electroradiography (as the pertinent methods are referred to generally) to convert the information content of an X-ray beam which has penetrated the object to be imaged, into electric charges and then to concentrate and fix the latter on paper or a plastic film. In medical electroradiography, the further requirement of high sensitivity is added to the requirements found in copying technology, since the equipment developed for diagnostic purposes already has a sensitivity which corresponds to that of X-ray films with intensifier film, and since the patient should not be exposed to a radiation dose higher than heretofore. Due to this sensitivity requirement for the method, xeroradiography, which was developed from xerography, has been eliminated for general application in medical diagnostics.
Another method, so-called high pressure ionography, works according to the principle of an ionization chamber. The charge carriers, which are generated when X-rays pass through a gas space, are collected on a film. This known method has high sensitivity and definition, but, technically, is a less satisfactory solution. For, in order to obtain sufficiently high absorption of the radiation in the gas volume, a gas with a high atomic number, for instance, expensive xenon, must be used. Furthermore, it must be present in the chamber at an elevated pressure of, for instance, 5 bar. This places stringent requirements on the design of the chamber. In addition, the imaging chamber must be opened after each exposure to remove the charged film. The technique required is therefore relatively expensive and the total picture taking process requires considerable time.
Another method is the so-called low pressure ionography (Phys. Med. Biol. 18 (1973), pages 695 to 703). In this method, the external X-ray photo effect of a solid state photocathode is utilized for generating electric charge carriers. The emitted photo electrons are subsequently multiplied in the gas space of a suitable chamber by means of a Townsend discharge to such an extent that a developable electrostatic image is generated on paper or plastic foil. If, instead of these foils, an electroluminescent fluorescent screen is used for collecting the charges, then it is also possible with this method to display image sequences, i.e., moving pictures. Such a method is known as electrofluoroscopy. A known example of such, therefore is the X-ray image intensifier.
If a suitable filling gas which can be at atmospheric pressure is used in the chamber of such a photocathode, multiplication factors of 10.sup.4 can be achieved without difficulty. Because of the mismatch of the depth of penetration of the X-rays to the range of the emitted photo electrons, which is about 100:1, solid, plane photocathodes provide a quantum yield of about 0.5 to 1%. Quantum yield is understood here to mean the number of photo electrons emitted per incident X-ray quantum. With the quantum yield of the known photocathodes, it is therefore not possible to meet the requirements of medical technology as to sensitivity and resolution.